This invention relates generally to rechargeable batteries of the type which employ zinc as an anode active material and a metal oxide or hydroxide as an active cathode, and particularly to electrolytes for use with such batteries.
Because of the solubility in acid or neutral media of metal oxide and hydroxide cathode active materials, it is necessary to employ alkaline solutions as the electrolyte in such batteries. Typical examples of these batteries are silver oxide-zinc systems, nickel oxide-zinc systems, and manganese dioxide-zinc systems.
Conventionally batteries of the type just described employ solutions of sodium hydroxide or potassium hydroxide as an alkaline electrolyte. Though the performance of the oxide or hydroxide cathode with these electrolytes is satisfactory, the same is not true of zinc anodes, especially in the case of rechargeable batteries. This is due to the fact that upon discharge of the zinc anode, zinc oxide and hydroxide products are formed at the anode. These products are soluable in the electrolyte which results in a major dissolution of the anode discharged products. When charging is subsequently employed, difficulties in the mass transfer of the zinc arise since most of the zincate is not within the porous body of the zinc anode but rather outside in the electrolyte body and in the separators due to its solubility. This, in turn, leads to a tendancy for electrodeposition to occur at the outer surfaces and points of the zinc anodes resulting in needle-like and tree-like dentritic deposits which are usually non-adherent and penetrate the separators in the battery. After a number of cycles such deposits progressively lead to failure of the battery through internal short circuiting.
Another result of the solubility of the zinc anode product formed during discharge is the redistribution of the zinc within the cell, usually towards the lower part of the electrode due to the gravity field effect. With repeated cycling the zinc material deplates from the top portions and deposit heavily on the bottom portions of the anodes. This results in what is known generally as "shape change". These phenomena can also lead to cell short circuiting and capacity losses due to material non-adherance.
Modifications of the alkaline electrolyte have been described from time to time for certain purposes. For instance, the salting out effect of additions of sodium ethylate, boric acid and zincates has been suggested for removal of small amounts of silver ions entering the electrolyte from the positive plate in a silver oxide or silver peroxide-zinc battery. This has been done to protect zinc anodes from reacting with the silver ions or colloidal particles which would lead to their destruction by a "local action" corrosion mechanism. For example, U.S. Pat. Nos. 2,513,292 and 2,681,378 illustrate additions of a number of materials for salting out purposes. Here, sodium ethylate, zincates and boric acid is employed to control the silver migration problem. The electrolyte remains strongly alkaline and the concentration levels of the additives are relatively minor compared to that of the alkali hydroxides. Indeed, an excess of hydroxide on the order of some 10.4 to 12.8 equivalents per liter is used. This, however, does not significantly suppress zinc or zinc alloy anode dentritic electrodeposition nor "shape change" from occuring during cycling in the battery.